Reconstituted membranes containing purified acetylcholine receptor (AcChR) as the only protein component (J.M. Gonzalez-Ros, A. Paraschos, and M. Martinez-Carrion (1980) Proc. Natl. Acad. Sci. 77, 1796) provide a unique " in vitro" system to accurately study receptor mechanism. This experimental system will be used: 1) To determine the role of specific lipid types and/or lipid-AcChR interactions on AcChR function. This first objective will imply an initial detailed study on the bulk lipid composition as well as the "boundary" lipids. A systematic alteration of the lipid composition in the reconstituted AcChR system will be accomplished. Finally, "normal" receptor functionality will be correlated with both physical and chemical features of the lipid phase. 2) To develop a rapid spectrofluorometric technique to monitor AcChR-mediated cation translocation events within the physiological time scale. This technique will be based on the trapping of water soluble, non-permeable fluorophores within the reconstituted AcChR vesicles and subsequent quenching by externally added T1 plus.